The present invention relates to the manufacture of printed circuit boards and more particularly to a method for temporarily sealing holes in the printed circuit board laminate during processing.
In the manufacture of printed circuit boards, a photoresist is used to transfer the outline of the circuit into the copper surface of the board. The name photoresist defines the dual functioning nature of this material. First it is a photo polymer whose chemical properties are changed by exposure to ultraviolet radiation. That exposure is done selectively through a mask outlining the circuit being defined. The dual functioning comes into play after developing the photo-polymer, where the soft unwanted areas are washed off the copper surface. What remains is a protective covering of hardened polymer only in those areas outlined by the exposure mask. In one application this protective covering resists the etching process so that only the copper left unprotected is etched away. When the resist is finally removed, the protected copper circuit lines underneath become the electrical conductors of the circuit board.
One real measure of the evolution of printed circuit board technology is the width of the copper circuit lines and the spacing between them. As the component density and circuits per square inch increase, the width of the circuit lines and the spaces between them must decrease. The current state of the art is 10 mil lines with 10 mil spaces. This geometry is ultimately determined by the process technology that allows the reliable fabrication of circuit boards within tolerances acceptable to the industry. In normal production a 10 mil wide circuit line can be controlled to within plus or minus 1 mil. If this line is spaced 10 mils from an adjacent line that may also vary by only 1 mil, there is little chance of having broken lines or short circuiting between lines. If, however, that line spacing geometry is reduced to 1 mil lines and spaces, the previous tolerance is unacceptable and the process technology must be advanced to achieve and maintain a tighter tolerance.
The most reliable and efficient method for connecting circuits on opposite sides of a circuit board is the use of the plated through hole. Before the circuit pattern is etched into the copper surfaces of the circuit board, the necessary interconnecting points between the two sides are first located and a hole drilled through the board at that position. In a complex circuit, there may be hundreds of holes each with its own specification and tolerances, making the precision, quality and cleanliness of each drill hole critical. Generally, after the circuit pattern has been defined, a copper conductor is plated inside along the wall of each hole connecting the copper circuits on one side to the other. To provide a good plated connection this hole must be clean and free of any photoresist or other contaminant. The plated connection must be almost perfect because each hole diameter is reduced by the thickness of the copper plating. The remaining hole must be large enough for the leads of the circuit board's components to be inserted into it but not too large to be filled with solder for a final connection.
Two methods are commonly used for applying photoresist to the copper surface of a circuit board. One of these is coating and the other is lamination. In coating, a fluid containing the photopolymer dissolved in solvent is applied to the copper surface in a thin uniform layer. The solvent is evaporated away and a uniform film of photoresist is deposited onto the copper surface. In lamination a previously coated and dried film of photoresist on a carrier web is bonded to the copper surface using heat and pressure, after which the carrier web is stripped away.
Most of the circuit boards produced today use the dry film method primarily for these two reasons. First there is no solvent fluid to cause safety, personnel, environmental or disposal problems. Secondly, there is no liquid photoresist to get inside the drill through holes to contaminate them and jeopardize the integrity of the plated through connections. These two advantages of dry film over coating are substantial but they are obtained at a price. One price is economic as dry film is about three times as costly per square foot as a coated photoresist. The other price, far more costly is technological. Dry film has not been able to be produced reliably below a thicknes of one mil. In order to reduce the line spacing geometry so that circuit density can be significantly increased, it is necessary to reduce the thickness of the photoresist to around 0.1 to 0.2 mils. A reliable well adhered photoresist can only be applied in that thickness range by using liquid coating technology. Handling the solvents associated with liquid photoresists has already been solved, however, before any liquid coating technology can be reliably used, the holes in the circuit board laminate must be temporarily sealed.
There are three major problems to be overcome in the temporary sealing of the holes in printed circuit board laminates before they are coated with photoresist. First, the sealing material must prevent any significant amount of photoresist from entering the hole. Secondly, the sealing material must be chemically inert, mechanically rugged and adhered strongly enough to maintain its sealing function during the subsequent processing steps. Finally and most importantly, the sealant material must be completely removed from every hole without leaving any contamination or residue on the hole wall.
Various methods for temporarily sealing the holes have been proposed. Early work described in U.S. Pat. No. 2,965,952 related to filling the holes with an inert material such as a protein substance. As pointed out in this patent, the process of filling and later cleansing of the plated holes was very time consuming and required a plating process in order to insure that the filling material would reside only in the holes and not on the surfaces of the plate. U.S. Pat. No. 2,965,952 attempts to solve the filling and cleansing problem by employing decalcomanias as a resist to the action of the etching solution. The decalomanias comprised a plurality of adhesive-backed strips that extended over the holes in the printed circuit board. This procedure required accurate registration of the protective strips and was also time consuming. U.S. Pat. No. 3,725,215 described a filler masking of small apertures in the printed circuit boards. The process utilized a photo-hardenable material that was placed in the holes with a backing sheet to prevent the material from falling through. After filling the holes, the photo-hardenable material was exposed to radiant energy to solidify it. Again, this process was time consuming and required care that the photo-hardenable material did not extend beyond the perimeter holes themselves.
It is accordingly a general object of the present invention to provide an improved method for temporarily sealing holes in printed circuit board laminates.
It is a specific object of the invention to provide a method for temporarily sealing printed circuit board holes utilizing a deformable sheet material that can be deformed into the holes to form protective sealing plugs therein and that can be easily removed from the printed circuit board after conventional processing of the board.
It is still another object of the present invention to provide a thermodeformable sheet material that can be thermally deformed under a pressure differential established across the thickness of the sheet material to deform the material into the printed circuit board holes.
It is a further object of the invention to provide a cost effective process for temporarily sealing printed circuit board holes in which the sealing material can be re-used a number of times.
It is a feature of the invention that the method can be practiced using commercial, readily available materials.
It is another feature of the invention that the deformable sheet material used in practicing the invention can be re-used a number of times without requiring any additional processing after the deformed sheet material has been removed from the printed circuit board.